Hollow fiber membrane and braided tubular support therefor

ABSTRACT

An asymmetric membrane comprising a tubular polymer film in combination with a tubular braid on which the film is supported, requires the braid be macroporous and flexible, yet sufficiently strong to withstand continuous flexing, stretching and abrasion during use for microfiltration (MF) or ultrafiltration (UF). The specifications for a braid of a long-lived membrane are provided. A membrane is formed by supporting a polymer film in which particles of calcined α-alumina are dispersed, on the defined tubular braid.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of application Ser.No. 08/886,652 filed Jul. 1, 1997 to issue as U.S. Pat. No. 5,914,039 onJun. 22, 1999.

FIELD OF THE INVENTION

[0002] This invention relates to a braided tubular support for a film ofpolymer which functions as an asymmetric semipermeable membrane inmicrofiltration (MF) and ultrafiltration (UF) applications. The braidedtube is no more than about 3 mm in outside diameter and relies on thepolymer film to imbue the fiber membrane product with sustainable highflux along with sufficient abrasion resistance such that a skein offibers (also referred to as a “module”) can operate in a commercialfiltration application for several months without the formation ofpin-holes.

BACKGROUND OF THE INVENTION

[0003] U.S. Pat. No. 5,472,607 to Mailvaganam Mahendran et al disclosesa hollow fiber semipermeable membrane in which a tubular macroporoussupport is superficially coated on its outer surface with a thin film ofpolymer, most preferably of polyvinylidene difluoride. The tubular braidis flaccid but other details of the structure of the braid are notspecified. For example, the effect of characteristics of the materialforming the braid were not known; nor was the effect of a cross-sectionwhich was not truly circular, i.e. having “cylindricity” substantiallyless than 1.0. The term “cylindricity” (sometimes referred to as“roundness”) refers to how perfectly the circular cross-section of thetubular support matches the geometry of a true circle drawn tocorrespond to the mean diameter of the braid, a perfect match being 1.0.It was therefore not known at that time, how critical the physicalcharacteristics of a preferred braid were to the performance of a hollowfiber membrane using the braid.

[0004] In commercially available braid, made with conventional braidingequipment from commercially available yarn, there were numerous “breaks”in the fiber; also, accumulation of clumps of broken filaments, referredto as “fuzz”, braided into the cylindrical wall of the braid, resultedin weak spots in the polymer film coated onto the surface; and brokenfilaments, referred to as “whiskers”, protruding from the surface of thetubular braid, resulted in too-thick domains of polymer which wereconcentrated around the whiskers; and, when the domain was nottoo-thick, whiskers have a proclivity to initiate pin-holes.

[0005] Further, if the open weave of the braid provided either too highor too low a braid porosity as measured by resistance to air flow, thefiber membrane formed was unusable in a commercial installation. Tooopen a weave resulted in the braid being embedded, that is, enclosed byand firmly fixed in the polymer which also infiltrates into the bore ofthe braid; thus, too open a weave results in greatly reducedpermeability. Too tight a weave results in the polymer not beinganchored sufficiently well on the surface; this increases the likelihoodthat, in service, the polymer film will be peeled from the braid. Whenoperating flux was excellent, portions of the polymer film weresometimes found to have been peeled away when the fibers were backwashedwith clean water or other fluid medium, whether water or permeate, underpressure; or portions of the film were “blown off” the surface of thefibers when their lumens were pulsed with air under pressure. Even withthe best braid produced under controlled conditions, shrinkage duringusage in an aqueous medium varied unpredictably. This resulted in tautfibers which were prematurely fouled because they were unable to movesufficiently to stay clean or rub against each other. If too taut, thefibers are broken before they are fouled, or torn from potting resin inthe header. Particularly because it is essential for best performance,and to shed contaminants from the surfaces of the hollow fibermembranes, that a skein of fibers operate with “slack” fibers, thestructure of the braid needs to survive repetitive twisting, and it wasnot known what physical characteristic(s) of the braid was conducive tosuch survival. A cylindricity less than 0.8 resulted in a polymer filmwith unacceptable variations in thickness resulting in non-uniform fluxand zones which were too easily fouled.

[0006] The goal to achieve great strength led one to choose a highstrength yarn, e.g. of glass, aramid or other high modulus material, tobenefit from its high strength and stability. For example, a braid wovenfrom glass multifilaments, has insignificant maximum extension at break,less than about 5%, and is essentially non-shrinkable. However, inpractice, braided fibers woven with such stable high modulus yarns arenot desirable as they provide inadequate adherence of film to thesurface of the braid, attributed to the negligible moisture regain ofsuch fibers, and when wet, the braid is too fragile for prolongedservice. In particular, it was not known that a certain range ofmoisture regain in the material of the braid was essential for optimumoperation of hollow fiber membranes coated with a hydrophilic polymerfilm and operated in an aqueous or alcoholic environment. The moistureregain is the percentage of moisture in a textile material brought intoequilibrium with a standard atmosphere after partial drying, calculatedas a percentage of the moisture-free weight.

[0007] The goal to anchor the polymer film non-removably, and to achievea high “bubble point” in a membrane with no defects (such as pin-holes)was not identified in the '607 disclosure because the factors whichaffected the goal were not known. The “bubble point” refers to thepressure under which a stream of air escapes through the largest pore ina wall of a defect-free membrane which has desirable flux. Further, theimportance of stability of the structure of the braid during operation,particularly the effect of shrinkage, was not known.

SUMMARY OF THE INVENTION

[0008] It has been discovered that certain physical characteristics of atubular braid are critical to the formation of a desirable hollow fiberMF or UF, that is, liquid-separation membrane which is stable andstrong, yet has an essentially trouble-free useful life and anacceptable, desirably high, permeability.

[0009] It is therefore a general object of this invention to provide atubular braid support for an asymmetric membrane, woven from yarn madewith synthetic resinous filaments essentially insoluble in the solventin which the membrane-forming polymer is dissolved, the braid having astable heat-pre-shrunk length which is in the range from about 1% to 20%less than its unshrunk length, preferably so that, irrespective of thematerial forming the fibers, when the pre-shrunk braid is stretchedlongitudinally, it has “give”, that is, the extension at break is atleast 10%, preferably in the range from 10% to 30%, and more preferablyabout 20%.

[0010] It is a specific object of this invention to provide aheat-pre-shrunk tubular braid made with specified patterns, usingcarriers carrying yarn having defined number of filaments, ends, denier,and picks, under conditions which control the porosity (measured aspermeability to air) of the braid, such controlled porosity serving toanchor a polymer film non-removably on the surface of the tubular braid.

[0011] It is another specific object of this invention to provide, in aflexible macroporous tubular braid support for an outside-in hollowfiber asymmetric membrane having a tubular film of synthetic resinousmaterial supported on the outer circumferential surface of the braidwithout the support being embedded in a thin film having a wallthickness of less than 0.2 mm, the improvement comprising, 16 to 60separate yarns, each on its own carrier, each yarn being multifilament150 to 500 denier (g/9000 meters) yarn, each multifilament being madewith from 25 to 750 filaments, each filament being from 0.5 to 7 denier.From 1 to 3 multifilament ends constitute a yarn, and the individualends are most preferably not plied together, but lie linearly adjacentto each other until taken up in the “fell” of the braid being woven. Thebraid being woven has from 30 to 45 picks (crosses/inch). The higher thedenier of the filaments, the fewer the filaments used, but the braidwall thickness is maintained in the range from about 0.2 mm but lessthan three times the diameter of the yarn from which the braid is woven,preferably less than 1.0 mm. The air permeability of the braid ofsynthetic resinous yarn is in the range from about 1 to 10 cc/sec/cm² ata differential pressure of 1.378 kPa (0.2 psi); and the moisture regainis in the range from about 0.2% to 7% by weight (wt). The finished fibermembrane is coated with a thin polymer film having a thickness in therange from 0.05 mm to 0.3 mm, most preferably less than 0.1 mm thick.The film has an annular peripheral barrier layer or “skin”circumferentially integral with successive microporous layers in thefilm, each layer contiguous with a preceding layer, the layers includingan outer annular layer, an intermediate transport layer, and an annularinner layer.

BRIEF DESCRIPTION OF THE DRAWING

[0012] The foregoing and additional objects and advantages of theinvention will best be understood by reference to the following detaileddescription, accompanied with schematic illustrations of preferredembodiments of the invention, in which illustrations like referencenumerals refer to like elements, and in which:

[0013]FIG. 1A schematically illustrates a “diamond” pattern in a tubularbraid.

[0014]FIG. 1B schematically illustrates a “regular” pattern in a tubularbraid.

[0015]FIG. 1C schematically illustrates a “Hercules” pattern in atubular braid.

[0016]FIG. 2 is a cross-sectional elevational view along a longitudinalaxis, of a coating nozzle used to form the thin non-supporting filmmembrane on the braid.

[0017]FIG. 3 is a cross-sectional end view of a hollow fiber membrane ofthis invention schematically illustrating the radially disposed annularzones which extend longitudinally axially over the length of themembrane, and showing how the tubular non-self-supporting film issupported on the braid without being embedded therein.

[0018]FIG. 4 is a cross-sectional view with greatly enlarged dimensions,to illustrate the dimensional relationships of pores in the componentlayers of the braid-supported membrane which pores make the membrane soeffective, particularly for microfiltration and ultrafiltration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0019] Details of a hollow fiber membrane are presented in theaforementioned '607 patent and Ser. No. 08/886,652 application, thedisclosure of each of which is incorporated by reference thereto as iffully set forth herein. A preferred tubular braid is woven with yarn,the denier of which is chosen with consideration of the outside diameterof the braid on which the polymer film is to be coated, and whether themembrane is to be used for MF or UF. A desirable air-permeability for aUF membrane to provide drinking water, is in the range from about 5 to25 LMH/kPa (liters/m²/kPa/hr) or 20 to 100 GFD/psi (gals/ft²/day/psi),preferably from about 7.4 to 18.5 LMH/kPa (30 to 75 GFD/psi), measuredwith RO (reverse osmosis) water; a desirable permeability for a MFmembrane used to filter municipal sewage and provide clean water is inthe range from 10 to 50 LMH/kPa (40 to 200 GFD/psi), typically about12.5 to 25 LMH/kPa (50 to 100 GFD/kPa), measured with RO water. Atypical defect-free fiber has a bubble point in the range from about 140to 280 kPa (20 to 40 psi). For a UF membrane it is desirable to have abubble point in the range from 13 to 40 kPa (2 to 6 psi), preferablyabout 35 kPa (5 psi) to emphasize the importance of a defect in a fiber;for a MF membrane it is desirable to have a bubble point in the rangefrom 6 to 20 kPa (1 to 3 psi), preferably about 13 kPa (2 psi), for thesame reason.

[0020] The structure of the tubular braid is determined by the machineused to weave the braid which is formed of intertwined, spiral yarns sothat its thickness is less than three yarn diameters, and the yarnorientation is helical. The braided tube my be woven on either verticalor horizontal tubular braiding machines, the former being preferred. Amachine includes a track plate provided with intertwining tracks, pluraltube or bobbin carriers for the yarn capable of moving counterclockwiseor clockwise along the tracks for braiding, a former and a take-updevice. Bobbins are flanged tubes used for yarns which are difficult tohandle. Yarns from bobbins mounted on the bobbin carriers are braided asthey are guided to a gathering guide disposed above the center of thedisk. Each bobbin carrier is rotated by a drive gear disposed under thetrack plate while it moves along the tracks. The ratio between themoving speed of the bobbin carriers and the braid drawing speed can bechanged by changing the gear ratio, so that the braids may differ fromeach other in the angle of the strands. Different interlacings, or weavepatterns, can be achieved by controlling the motion of the yarncarriers. By controlling the take-up rate, the angle of the braid can becontrolled. It is essential that the yarn tension be controlled toprovide uniform tension so as to form a uniform braid. Machines formaking the tubular braid and the method of making it are well known andform no part of the invention. If desired, axial reinforcements may beprovided by using a third system of yarns which can be inserted betweenthe braiding yarns to produce a triaxial braid. Such reinforcement istypically found unnecessary.

[0021] A typical tubular braid is made from two sets of yarns or endswhich are intertwined. Preferred materials are polyesters and nylons inyarn which is most preferably in the range from about 200 to 400 denier(g/9000 meters), with from 40 to 100 filaments having a denier in therange from about 3 to 6. The braid is preferably woven with from 16 to28 carriers with from about 36 to 44 picks (crosses/inch) to have anoutside diameter in the range from about 1.5 mm to 2.5 mm and a wallthickness in the range from about 0.15 mm to about 0.50 mm, mostpreferably about 0.3 mm.

[0022] The pattern in which the braid is woven is not narrowly criticalprovided the porosity is maintained within chosen limits, and though a“Regular” or “Hercules” braid is usable, a “Diamond” pattern is mostpreferred. Referring to FIG. 1A, there is schematically illustrated adiamond braid having an alternation of one yarn passing above and thenbelow the other yarns (1/1). FIG. 1B illustrates the regular braidpasses above two and below two in a repeat (2/2). FIG. 1C illustratesthe Hercules braid which has a structure of 3 up, 3 down (3/3).

[0023] The load at break of a preferred heat-shrunk braid is at least 50lb-force, preferably from 444 to 888 Newtons (100 to 200 lb-force),recognizing that the heat-pre-shrunk synthetic resinous braid has astable length which is in the range from 1% to 16% less than itsunshrunk length.

[0024] The critical importance of providing a stable heat pre-shrunklength is because the outer surfaces of taut fibers in a skein (tautbecause of shrinkage during use) become so fouled that they areineffective to filter. Further, stresses on taut fiber membranes stressnot only the tubular braid but the overlying polymer film. Undue stresson the braid results in breakage, typically near the ends of the fibermembranes, where they are potted in headers; and undue stress on thepolymer film diminishes its adherence and increases its susceptibilityto peeling from, or sloughing off the surface of the braid.

[0025] Though a “shrink test” is commonly conducted on yarns by heatshrinking in water at 98° C. via a Texurmat boil off; or, in dry air at177° C. with 0.045 gf/dtex tension for 2 min (DuPont); or, in dry air at190° C. with 0.135 gf/d for 30 sec (Monsanto), to date there has been noreason to heat pre-shrink any tubular braid of synthetic resin, prior toits being coated with polymer. More particularly, since a braid wovenwith glass fiber is essentially non-heat-shrinkable, there has been noreason to provide a stable length of a polyester or nylon tubular braidby pre-shrinking it so that its shrunk length is about 84% of itspre-shrunk length at the same time ensuring that the braid retains atleast 95% of its tensile strength.

[0026] Heat-shrinking in dry air, referred to as Testrite tests, ofpolyester and polyamide tubular braids to obtain most preferably fromabout 16% to 18% shrinkage, may be acheived in an electric furnace at232° C. for 29 sec.

[0027] The denier of the yarn and structural characteristics of thebraid determine the liquid and gas permeability. The liquid permeabilityof the braid is at least one order of magnitude (that is, more than 10times) greater than the permeability of the polymer film. Thus the weaveof the braid is so open that it presents an insubstantial barrier to gasflow.

[0028] Permeability to air of preferred polyester (“PE”) and nylon(“NY”) braids, determined by ASTM Standard “Air Permeability of TextileFabrics D 737-96” are measured for a differential pressure of 1.38 kPa(0.2 psi). These are listed in the following Table 1 under “@0.2 kPa”.Also listed are permeabilities “@0.02 kPa” (0.029 psi) which areobtained by extrapolation of the data curve obtained with measurementsat 0.2 kPa, in the appropriate range: TABLE 1 permeability Sam- I.D.O.D. length area flowrate cc/sec/cm² ple mm mm mm cm² cc/sec @ 1.38 kPa@ 0.2 kPa PE-1 0.76 1.55 30 1.461 10.36  7.09 1.03 PE-2 1.02 1.89 281.663 7.05 4.24 0.62 PE-3 1.13 2.04 26 1.666 5.00 3.00 0.44 PE-4 0.761.89 25 1.484 1.52 1.02 0.17 NY-1 1.00 2.10 27 1.781 3.94 2.21 0.32 NY-20.89 1.86 27 1.578 4.29 2.72 0.37 NY-3 1.28 2.04 23 1.474 3.93 2.67 0.37

[0029] The moisture regain values for polyester braids are in the rangefrom about 0.2% to 0.5% by wt, and for the above PE samples, are fromabout 0.4% to 0.5% by wt. For nylon braids moisture regain values are inthe range from 4% to 7% by wt, and for the above NY samples are fromabout 4% to 5% by wt. The structure of the tubular braid provides anouter surface which is uniquely configured to have a membrane's polymerfilm adhere to the surface sufficiently so as not to be detached whenthe membrane is backwashed; the polymer film is held by the upperportion of the wall of the braid without having the wall embedded in thefilm. The degree of adherence is affected to some extent by the affinityof the chemical composition of the polymer for the material of thebraid, but to a greater extent by the structure of the braid. Thepolymer film may be any polymer which provides a satisfactory asymmetricmembrane, and may be formed from a polyester, polyamide, polyolefin,polyamine, polyurethane, polysulfone or cellulose acetate, mostpreferably PVDF containing calcined α-alumina as disclosed in Ser. No.08/886,652, the disclosure of which is incorporated by reference theretoas if fully set forth herein.

[0030] The test of establishing whether adherence is satisfactory isdetermined by a Peel Test Procedure carried out on a Lloyd Instruments“Materials Tester” (LR K5 with a 50 N load cell) having a “German Wheel”(the “Tester” for brevity).

[0031] The “German Wheel” is used to execute a peel test of a coating ona flexible substrate, at 90° to the surface of the substrate. Eachsample is especially prepared according to the standard being used. TheGerman Wheel consists of a free running axle mounted wheel and a yokewhich receives the wheel and connects it to the load to execute a test.The face of the wheel contains a sharp angled slot into which one end ofthe coated substrate is inserted and folded back against the sharp edge.This creates a mechanical lock which holds the sample tight as itslength is drawn, coating side up, around the periphery of the wheel andpassed through a locking clamp. The clamp site is just beyond a regionwhere the coating tab length has been separated from the substrate. Thusthe flexible substrate is clamped and the coating tab length hangsfreely, in front of the clamp.

[0032] All tests are done on wet membranes, by slitting a six inch (6″)wet membrane longitudinally. One and one-half inch (1.5″) of membrane ispeeled from the braid. A bare one inch (1″) section of braid is insertedinto the angle slot and the rest of the braid is bent around the wheelsuch that the longitudinal slot is facing toward the wheel surface. Theangled slot anchors one end and the loose end is placed in the floatingclamp and tightened. Any slack is removed by the sample tensioningscrew.

[0033] The loose end of the peeled section of membrane is placed in theupper clamp of the Tester. Four inches of membrane are pulled off thebraid at a rate of 100 mm/min. The German wheel rotates freely to keepthe angle of peel constant. The material tester outputs a graph showingthe amount of force required to peel the membrane off the substrate. Theresults of the samples are averaged together and plotted on a graph. Theaverage maximum force of approximately the two inch section is recorded.

[0034] Tensile Strength of Each Sample is conducted as follows:

[0035] The wet samples of membrane obtained from the Peel Test areplaced in the clamps of the Tester. The clamps are placed one inchapart. The membrane sample is pulled apart at a rate of 100 mm/min. Theaverage maximum force for the samples is recorded along with thestandard deviation.

[0036] Cylindricity of the braid is determined by visual examinationunder a microscope.

[0037] The asymmetric film comprises a very thin “skin” overlying a moreporous structure in which the pores are in open communication with oneanother. Such a membrane may be used for filtering either aqueous ornon-aqueous solvents. For filtration of a solvent such as a primary orsecondary alcohol, a ketone or a hydrocarbon, the polymer film isdeposited from a solution of a solvent-resistant polymer such aspolyacrylonitrile (PAN) or polyetherether ketone (PEEK).

[0038] Referring to FIG. 2 there is shown a cross-sectional view of thecoating nozzle indicated generally by reference numeral 10, which, inaddition to limiting the amount of dope (polymer in solution) passingthrough the nozzle, meters the correct amount of dope over the surface,and distributes the metered amount uniformly over the surface of thebraid (not shown) as it is drawn longitudinally axially through thenozzle.

[0039] The nozzle 10 comprises an inner barrel 12 having an internalbore 13 through which the braid is advanced into an axial bore 14 of anipple 15 which is threadedly secured in the end 16 of the inner barrel12. The bore 14 provides a rounding orifice to help the braid to acquirea circular cross-section before it is coated with dope. The roundingorifice 14 has a diameter in the range from about 1% to 10% less thanthe nominal diameter of the braid. The barrel 12 with the nipple 15 isinserted in an outer barrel member 20 having a cylindrical base 25. Theouter barrel 20 is provided with a stepped inner axial chamber with alarger bore 22 and a smaller bore 23 provided with threads (not shown)near the end of the bore 23. A top-hat bushing 27 having a stepped axialbore 27′ is threaded into the smaller bore 23 until it compresses anO-ring 27″ in a groove between the end of the barrel 20 and the lowerportion of the bushing. A sizing die 28 having a sizing orifice 24 ispress-fitted in the stepped axial bore 27′. The sizing orifice ensuresthe circularity of the cross-section of finished hollow membrane, uponleaving the rounding orifice. As the dope-coated braid is advancedthrough the sizing orifice, it dresses the outside diameter of thepolymer-coated surface to provide the dope with a desired wallthickness, which upon being coagulated, yields a thin film membranewhich is no more than 0.1 mm thick.

[0040] The base 25 is provided with a lower port 21 and an upper port 26each in open communication with the stepped bores 22 and 23, so thatdope introduced into the port 21 can flow into the reservoir formedaround the inner barrel 12, by the stepped bores 22 and 23, and travellongitudinally axially in the direction in which the braid is drawnthrough the larger bore 22, and the smaller bore 23 displacing air asthe reservoir fills. When the dope having filled the reservoir flows outof the top port 26, it is plugged. The base 25 is removably secured withthrough-bolts (not shown) through the base 25 to a radially extendingmounting flange 29 having a longitudinal body portion 29′. The bodyportion 29′ is provided with an internally threaded axial bore so thatthe body portion 29′ can be secured coaxially in position, aligning therounding orifice 14 and the sizing orifice 24. By increasing ordecreasing the number of turns of the body portion 29′ the distancebetween the mouth of the orifice 14 and the orifice 24 can be varied.This distance is adjusted, depending upon the rate at which the braid ispulled through, the viscosity of the dope, and the thickness of the filmof dope to be coated on the braid before it is immersed in thecoagulant. In all cases, the distance is adjusted by trial and error, toprovide a film of dope on the circumferential outer surface of the braidonly sufficient to coat the braid superficially, and not enough to embedthe braid in the film.

[0041] To draw the braid through the orifice 24, a longitudinal tensionis maintained on the braid of at least 1 Newton but not enough todistort the voids in the braid so badly that they cannot return to anequilibrium state as they are being coated with dope. Because the braidis not embedded in the viscous polymer solution, only the outer surfaceof the braid is contacted with the dope so as to provide the braid witha dope- and polymer-coated outer surface.

[0042] It will now be evident that the coating nozzle 10 is aspecial-purpose nozzle specifically designed to provide a predetermineddistance between the rounding orifice 14 and the sizing orifice 24 whilea dope coated braid no larger than about 2.5 mm (nominal o.d.) isadvanced through both orifices sequentially. The amount of dope meteredinto the coating nozzle and the rate at which the braid is advancedthrough the rounding orifice are determined by trial and error such asone skilled in this art is accustomed to engage in under comparablecircumstances.

[0043] After the dope-coated braid leaves the sizing orifice, it is ledinto a coagulating bath, typically under and over a series of rolls, sothat the liquid coagulant held in the bath contacts the entirecircumferential surface of the coated braid. Because the polymer isinsoluble in the coagulant it does not penetrate the thin film formedand enter the lumen. Upon contacting the coagulant, the dope coagulates,yielding the desired thin film membrane. The bore of the fiber containsair at atmospheric pressure.

[0044] Referring to FIG. 3 there is shown in a diametricalcross-sectional view, much enlarged, a tubular braid indicated generallyby reference numeral 30 comprising a braid of woven yarn 31 having a“lumen” (inner bore) 32 . A thin film membrane, indicated generally byreference numeral 33, is self-adherently secured to the circumferentialouter surface 34 which is rough and uneven because it is formed by theinterwoven yarn which, in the range of thickness used and the number ofpicks in which it is woven, does not result in an even surface. Theessential characteristic of the thin film membrane 33 is that it issupported superficially, on the circumferential surface of the tubularbraid without the braid becoming embedded in the thin film. Thischaracteristic is evident in a photomicrograph which clearly illustratesthat the circumferential inner surface of the tubular braid's bore 32 isessentially free of polymer.

[0045] Referring to FIG. 4 there is schematically illustrated, moregreatly enlarged than in FIG. 3, the asymmetric thin film membrane 33,which when formed by being coagulated, is itself striated into anoverlying ultrathin barrier layer or “skin” 35 and three distinctlyidentifiable layers of pores, an outer layer 36, an inner layer 38 andan intermediate transport layer 37 between outer layer 36 and innerlayer 38, as schematically illustrated in greater detail in FIG. 4. Theskin is a very thin dense layer of polymer formed as the dope contactsthe coagulant. By reason of the manner in which the skin and each layeris formed from the same polymer, the layers have, in a radially inwarddirection from under the skin to the braided yarn 39 which defines thebore 32, progressively larger pores. As shown in FIG. 4, each “end” 39or yarn consists of a multiplicity of filaments 39′, and thecircumferential surface of the interwoven strands of yarn does notprovide a smoothly cylindrical surface. The skin is generally thinnerand the pores for a MF membrane are larger than those of a UF membranemade from the same polymer. The measured skin thickness (by electronmicroscopy) for particular films made for the braided membrane, is givenbelow to appreciate its thickness in relation to the pores of thelayers. The approximate ranges of sizes of the pores for preferred MFand UF membranes are given below: TABLE 2 MF, μm UF, μm Skin 35,thickness 0.1-1.5 1-4 Outer layer 36, avg pore diam 0.5-1.0 0.5-2  Intermediate transport layer 37* 2-6  5-10 Inner layer 38, avg pore diam10-40  10-150

[0046] In membranes, in general, the thickness of the skin is smallrelative to the thickness of the layers. The skin is thicker in a UFmembrane than in a MF membrane, and it would be even thicker in a ROmembrane (not measured). Though FIG. 4 is not to scale, by reason of themanner in which the membrane is formed, the thickness of the outer layeris generally smaller than that of the transport layer, which in turn, isnot as thick as the inner layer.

[0047] The approximate thickness of each layer in a MF and UF braidedmembrane are given in the following Table 3. TABLE 3 Thickness, averageMF, μm UF, μm Skin 35, 0.1-1.5 1-4 Outer layer 36  5-10 20-40Intermediate transport layer 37 30-50 40-80 Inner layer 38  100-1000 100-1000

[0048] The following examples illustrate the invention, but should notbe construed as limiting the invention which is defined in the appendedclaims.

EXAMPLE 1

[0049] Coating Braids with Different Properties with the Same Dope:

[0050] In the following examples two tubular braids A and B, made fromyarns of nylon 6/6 fibers, and upon initial examination havingproperties which are essentially the same except for the denier of thefilament, are each coated with a dope of poly(vinylidenefluoride) (PVDF)in N-methyl-2-pyrrolidone (NMP), containing a polyhydroxy alcoholhydrophilic additive and having a viscosity of 38,000 cps. The rate offlow of solution to the nozzle is adjusted so that the solution isflowed upon and around the periphery of the braid over a coatingdistance of 3 mm (0.125 inch). The braid, coated with the solution isthen pulled through a sizing die having a diameter of 2.5 mm, then ledinto a coagulation tank where the polymer solution is coagulated inwater to afford a semipermeable membrane about 0.06 mm thick supportedon the tubular braid which assumes an essentially circularcross-section. It is then pulled through a glycerine bath, dried andtaken up onto the reel of a winder. The coating conditions for eachbraid are the same, namely:

[0051] Bath Temperature 46° C. (115° F.)

[0052] WUS* 12.19 meters/min (40 ft/min)

[0053] The braids differed as follows: Braid A Braid B Yarn Denier 315420 Filaments 68 68 Denier/filament 4.6 6.2 Ends 1 1 Picks 44 44Cylindricity 0.9 0.9 Mean outside diam. 1.88 mm 2.01 mm Mean insidediam. 0.86 mm 1.06 mm Shrinkage 3.4% 3.4% Breaking strength, lb-f 5.937.68 Button** 2.15 mm 2.53 mm

[0054] Upon being tested for filtration, coated Braid B provided apermeability twice that of coated Braid A. Upon examination of thecoated braids, it is found that Braid A, made with lower denier yarn,gave a “looser” braid which allowed the dope to penetrate to the innerwall of the braid, embedding it, and leaving little on the outersurface, as is evident from the following: Braid A Braid B Coated meanoutr diam. 1.89 mm 2.15 mm Thickness of coating 0.005 mm 0.070 mm Meanwall thickness 0.520 mm 0.475 mm Flux @ 15 psi 171.9 usgfd 383 usgfd

[0055] A photograph of a cross-section of the braided MF membrane, madewith an electron microscope, shows the film membrane overlying the braidto be about 0.05 mm thick and the braid is not embedded in the film. Thethickness of the skin 35, and each individual layer 36-38 will dependupon the conditions under which the film is made. Measurements made in avertical plane through the circumference, across the wall of the film,provides the following data on pore sizes: Section μm Skin thickness 0.8Outer layer 36* 0.781 Intermediate layer 37* 3.9 Inner layer 38* 14-32

[0056] The braided membrane was used to form a MF filtration modulehaving a construction described in U.S. Pat. No. 5,783,083 to Mahendranet al. The water permeability measured under 67 kPa (5 psi suctionpressure) and 22° C. is found to be 170 LMH (100 USgfd).

EXAMPLE 2

[0057] Comparison of Braids Made with Polyester and Glass Fiber Yarnsand an “ADC” Membrane:

[0058] A dope, code ADC, is made up similar to the PVDF-in-NMP solutionused in Example 1 hereabove, with 16 parts PVDF; 81 parts NMP; 2 partsHPVA; and 1 part LiCl; having a viscosity of 56,000 cps, and is fed to anozzle through which tubular braids of glass fiber and polyester areadvanced to prepare fibers which are substanitally identical except forthe material of the yarn from which the braids are made. As before, theof flow of dope adjusted so that the solution is flowed upon and aroundthe periphery of each braid over a coating distance of 3 mm (0.125inch), pulled through the same sizing die, coagulated in water to afforda thin semipermeable membrane 0.05 mm thick, supported on the braid,then pulled through a glycerine bath. Each braided MF membrane has ano.d. of about 1.88-1.92 mm, and cylindricity of about 0.9, the i.d. ofeach being about 0.9 mm. Each coated braid is taken up onto the reel ofa winder and used to make skeins.

[0059] The skeins, each having an area of 130 ft² are placed in MFservice in a reservoir of water contaminated with leachate from aland-fill site. The COD of the leachate is in the range from 1000 to1500 mg/L. Air in an amount in the range from 400 to 450 m³/hr isprovided at the base of each skein. After six months service under usualoperating conditions and identical back-flushing procedures, it wasfound that every skein made with glass fiber braid had suffered from 2to 20 broken fibers.

[0060] When skeins made with fibers of polyester braid are placed in thesame service as the fibers of glass fiber braid above, under identicaloperating conditions and the same back-flushing procedures, it was foundthat after six months service, not a single fiber of polyester braid wasbroken.

EXAMPLE 3

[0061] A dope is made up similar to the PVDF-in-NMP solution used inExample 1 hereabove, except that it is made up with the followingcomponents in the relative amounts (parts by weight) set forth:N-methyl-2-pyrrolidone (NMP) 82; polyvinylidene fluoride (PVDF) 15;calcined α-alumina particles (“α-Al”) 2; 50% hydrolyzed polyvinylacetate (HPVA) 1; for a total of 100 parts.

[0062] 70 g of calcined α-Al particles having an average primaryparticle size of about 0.4 μm are weighted in a flask to which 2787 g ofNMP is added and thoroughly mixed in a Sonicator® for at least 1.5 hr,to ensure that agglomerates of primary particles are broken up so as toform a suspension in which individual primary particles are maintainedin spaced apart relationship with each other in the NMP. The suspensionis milky white, the white color being contributed by the white calcinedα-Al. To this suspension is slowly added 525 g of PVDF having a numberaverage mol wt of about 30,000 Daltons while stirring at high speeduntil addition of the PVDF is complete. During the addition of the PVDFthe milky white color of the suspension changes first to pink, then toyellowish brown, at the end to grey/brown. Since PVDF dissolved in NMPproduces no color change, and the milky white color of the suspension isattributable to the α-Al particles, the changes in color provideevidence of a reaction between the calcined α-Al or a base present inthe calcined alumina.

[0063] When the grey/brown color of the NMP/PVDF/α-Al complex insuspension is stable and does not change upon standing for a sustainedperiod in the range from 4 hr to 24 hr, 118 g of a 30% solution of 50%HPVA containing 1.6-1.7% sulfuric acid in NMP is added to form a dopewhich is stirred overnight. The dope is then degassed either by lettingit stand for 24 hr, or by centrifuging it. The viscosity of the degasseddope is about 14,500 centipoise (cp).

[0064] The dope formed is fed to a nozzle through which Braid B used inExample 1 above is advanced at about 12.2 meters/min (40 ft/min), andcoated at a pressure of 274 kPa (25 psig) over a coating distance of 3mm (0.125 inch). The coated braid is sized in a sizing die having adiameter of 2.55 mm, then led into a coagulation tank where the polymersolution is coagulated in water to afford a semipermeable membrane about0.13 mm thick, supported on the tubular braid which has a cylindricityof about 0.9. This coated braid was then quenched by immersion insequential first and second coagulation baths of water, each at 47° C.(116° F.), and finally through a glycerine bath before it is taken uponto the reel of a winder. In tests, it is found that the braided MFmembrane provides excellent results.

[0065] After a section of the braided membrane was washed overnight incold water, its water permeability is determined by measuring its fluxwhich is found to be 6 LMH/kPa or, permeability of 25 GFD/psi measuredat 5 psi. After another section of the braided membrane, it is treatedwith an aqueous solution containing 2000 ppm of sodium hypochlorite(NaOCl). Water permeability of the NaOCl-treated membrane was found tobe 12 LMH/kPa measured at 35 kPa (50 GFD/psi measured at 5 psi). In eachcase, the pore size measurements and molecular weight cut-offmeasurements provide evidence that the pores in the film are suitablefor microfiltration.

We claim:
 1. A liquid separation asymmetric membrane comprising atubular braid support synthetic resinous yarn woven so that said braidhas a stable heat-pre-shrunk length which is in the range from about 1%to 20% less than its unshrunk length and an air permeability less than10 cc/sec/cm² at 1.378 kPa.
 2. The tubular braid support of claim 1wherein said braid, stretched longitudinally, has an extension at breakof at least 10%.
 3. The tubular braid support of claim 1 wherein saidbraid is woven with from 16 to 60 carriers, each using multifilament 150to 500 denier yarn, each multifilament being made with from 25 to 750filaments, each filament being from 0.5 to 7 denier, said yarncomprising from 1 to 3 multifilament ends.
 4. The tubular braid supportof claim 3 wherein said braid has from 30 to 45 picks (crosses/inch). 5.The tubular braid support of claim 4 wherein said filaments formed froma synthetic resin selected from the group consisting of polyester,polyamide, polyolefin, polyamine, polyurethane, polysulfone andcellulose acetate.
 6. The tubular braid support of claim 5 wherein saidbraid has a wall thickness in the range from 0.2 mm to less than threetimes the thickness of said yarn.
 7. The tubular braid support of claim5 wherein said synthetic resin is selected from the group consisting ofpolyester and polyamide, said braid has sufficient strength to exhibit aload at break of least 50 lb-force.
 8. The tubular braid support ofclaim 7 wherein said braid has an inside diameter more than 0.5 mm, anoutside diameter less than 3 mm and a wall thickness in the range fromabout 0.2 mm to about 1.0 mm.
 9. In a flexible macroporous tubular braidsupport for an outside-in hollow fiber asymmetric membrane having atubular film of synthetic resinous material supported on the outercircumferential surface of said braid without the support being embeddedin said film which has a wall thickness of less than 0.2 mm, theimprovement comprising, from about 16 to 60 separate yarns, each on itsown carrier, each yarn using multifilament 150 to 500 denier (gm/9000meters) yarn, each multifilament being made with from 25 to 750filaments, each filament being from 0.5 to 7 denier, said braid beingwoven with from 1 to 3 multifilament ends at from 30 to 45 picks(crosses/inch).
 10. The tubular braid support of claim 9 wherein saidends are non-plied in each said yarn but lie linearly adjacent eachother until taken up to form said braid which is pre-shrunk to a lengthin the range from about 1% to 8% less than its unshrunk length.
 11. Thetubular braid support of claim 9 having a cylindricity greater than 0.8and a maximum extension at break of at least 10%.
 12. The tubular braidsupport of claim 11 having a wall thickness in the range from about 0.2mm to less than three times the diameter of said yarn, and a maximumextension at break of at least 20%.
 13. The tubular braid support ofclaim 11 wherein said separate yarns are woven in a pattern chosen fromRegular, Diamond and Hercules.
 14. In an outside-in hollow fiberasymmetric semipermeable membrane comprising, (i) a macroporousforaminous tubular support means having an outer surface; and, (ii) apolymeric film of a reaction product of (a) a complex of polyvinylidenedifluoride (PVDF) with calcined α-alumina particles, and (b) ahydrophilic polymer adapted to impart hydrophilicity to said membrane;said particles having a primary particle size in the range from about0.1 μm to 5 μm being present in an amount at least 1 percent by weight,but less than 50 percent by weight, of said film; said film beingsupported by said outer surface, and said film having a peripheralbarrier layer or “skin” integral with successive microporous layershaving pores having an average diameter in the range from about 0.01 μmto about 0.3 μm, in open communication with each other, the improvementcomprising, a flexible macroporous tubular braid support comprising fromabout 16 to 60 separate yarns, each on its own carrier, each yarn usingmultifilament 150 to 500 denier (gm/9000 meters) yarn, eachmultifilament being made with from 25 to 750 filaments, each filamentbeing from 0.5 to 7 denier, said braid being woven with from 1 to 3multifilament ends at from 30 to 45 picks (crosses/inch).
 15. The hollowfiber of claim 14 wherein said macroporous foraminous tubular supportmeans support means has a stable heat-pre-shrunk length which is in therange from about 1% to 20% less than its unshrunk length and an airpermeability less than 10 cc/sec/cm² at 1.378 kPa.
 16. The hollow fiberof claim 15 wherein said filaments are formed from a synthetic resinselected from the group consisting of polyester and polyamide.
 17. Thehollow fiber of claim 15 wherein said braid, stretched longitudinally,has an extension at break in the range from 10% to 30%.
 18. The hollowfiber of claim 16 wherein said braid has an inside diameter more than0.5 mm, an outside diameter less than 3 mm and a wall thickness in therange from about 0.2 mm to about 1.0 mm.
 19. The tubular braid supportof claim 18 wherein said separate yarns are woven in a pattern chosenfrom Regular, Diamond and Hercules.